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I've read the Spark plug FAQ topic and clicked on a few of the links , but I'm still alittle confused . I'm having misfire/CEL on cold startup , but once I start driving the car is perfect. On cold startup , when the needle starts to fall from 1500 rpm to the 750 rpm it gets erratic inbetween and sometimes throws a misfire on all 4 cylinders , once the proper operating temp is achieved no more misfire or erratic tach issues .

I've been reading where some ppl switch back to the stock plugs and problem solved . Will I have to retune if i switch back to the stock plugs ?
Does Stock plugs limit limit the amount of power the car will make with a bigger turbo ?

I have seen some tuners mention that stock heat range plugs work best. I myself just switched from one-step colder plugs back to the oem plugs that came with the car. I was having misfire issues and somehow the gap had opened up a lot on the colder plugs (my biggest feeler gauge was .035 and it was loose). Didn't have to retune anything, but my tune is far from polished. Car runs like a raped ape now

i have a similar set-up as you and the exact same issue but i had mis-fire one 1 & 3. I cleared the code and it came back. i even had the 1500-750rpm thing, followed by misfire code during cold start up but perfect operation when warm. this all started the day after 3 back to back runs at the strip.. before that, it was normal operation.

car is 06sti w/gt35r & colder plugs...

i called my tuner (Doug at Topspeed) and he told me to change the plugs. i changed them and it is about 75% better now. the issue is not nearly as bad as it was. just let it idle about 15 seconds then drive off slowly. i have not been able to get rid of the surge thing when cold but the new plugs kept the mis-fire code away, and the surge is very much less pronounced than it was.

It depends on if you need colder plugs (each engine and tune is different). This is not a one size fits all change. If the plugs are fouling or misfiring cold you probably should go to a warmer heat range. If the plugs are running hot and your having detonation problems at high boost try a colder plug.

For performance applications, run the coldest plug you can that does not cause drivability problems. For daily drivers with moderate tunes almost without exception the stock plug is the way to go.

If you want to know if your plugs are too cold or too hot, you need to learn to read the plugs. For heat range you want to look at the discoloration band on the ground electrode. If it right near the tip the plug is too cold. If it is a couple mm back from the tip of the ground electrod your about right. If the whole ground electrode is discolored, the plug is too hot. (this is for normal plugs not sure how the newer plugs show color on their ground electrodes but it is probably the same)

i have a similar set-up as you and the exact same issue but i had mis-fire one 1 & 3. I cleared the code and it came back. i even had the 1500-750rpm thing, followed by misfire code during cold start up but perfect operation when warm. this all started the day after 3 back to back runs at the strip.. before that, it was normal operation.

car is 06sti w/gt35r & colder plugs...

i called my tuner (Doug at Topspeed) and he told me to change the plugs. i changed them and it is about 75% better now. the issue is not nearly as bad as it was. just let it idle about 15 seconds then drive off slowly. i have not been able to get rid of the surge thing when cold but the new plugs kept the mis-fire code away, and the surge is very much less pronounced than it was.

i have one step colder than stock.

car runs great and I have been running that way for about 4 months

Are you still having these issues ? or did you find a solution ?

Mine still does it if I don't let the car warm up properly , between 5-10 mins .